Method for reducing the silica content of alumina-containing materials of the clay type



June 7, 1960 H, SCHOENFELDER ETAL 2,939,764L

METHOD FOR REDUCING THE SILICA CONTENT 0F ALUMINA-CONTAINING MATERIALS 0F THE: CLAY TYPE Filed March 7, 1958 4 Sheets-Sheet 1 HERBERT scHof/w-ELDER HANS a/Nefeo ,ew Tnm mm Arrow/5K5 June 7, 1960 H, SCHOENFELDER ETAL 2,939,764

METHOD FOR REDUCING THE SILICA CONTENT 0F ALUMINA-CONTAINING MATERIALS 0F THE: CLAY TYPE HERBERT SCHOENFELDER HA N5 G/NSBE? wf www JM Afro/wem June 7, 19'60 H. scHoENFELDER ETAL 2,939,764 METHOD FOR REDUCING THE SILICA CONTENT OF ALUMIN-CONTAINING MATERIALS oF THE: CLAY TYPE 4 Sheets-Sheet. 3

Filed March 7, 1958 June 7, 1960 Filed March 7, 1958 H. SCHOENFELDER ETAL METHOD FOR REDUCING THE SILICA CONTENT OF ALUMINA-CONTAINING MATERIALS OF THE CLAY TYPE 4 Sheets-Sheet 4 HEERT SCHOENFELDER MN5 G//VSEG ATTORNEYS United States Patent O 2,939,764 METHOD FOR REDUCING THE SILICA CONTENT OF ALUMINA-CONTAINING vMATERIALS OF THE CLAY TYPE Filed Mar. 7, 1958, Ser. N0. 719,810 Claims. (Cl. 23-143) 'This invention relates to a method of decomposing alumina containing substances having high silicia acid contents; it relates more particularly to a method ofrenriching the alumina content and decreasing the silica contci;v of certain clays having an SiO; content highei than 4 o. i This application is a continuation-impart of pending a-pplication Serial No. 464,884, iiled on October 2,6, 1954, and now abandoned.

In the decomposition of substances containing alumina, use is frequently made of the so-called lime-soda process, in which the silicic acid present in the starting material is intended to be converted by sintering with lime -into calcium silicate, while the alumina is converted by soda (Na`2CO3) into soluble aluminate which is sub sequently recovered by leaching out the sintered product and processing the resulting solution to obtain alumina hydrate (Al2O3-H2o, A12O3.2H2O, uIICBV? ever, in this known process as well as in the Bayer process, a high silicic acid content Ain the starting materials must be avoided, since, otherwise, there is danger `that a part of the alkali (Na,K ions) will lbe lost for the formation of alkali metal alnminate, and be instead used up `for the formation of alkali metal silicates.

A great number of Wmethods have been .proposed in the art for reducing the silicic acid Fcontent of'the starting materials prior `to yu sing them as a first s tep of preparing suitable starting materials `in the above-mentioned processes for `the decomposition of alumina containing anaterials. `These methods frequently employ a preliminary treatment of `the s t-arting materials by heat treatment ,at elevated temperatures, and subsequent leaching with `capstic soda (sodium hydroxide) solutions orrsoda (sodium carbonate) solutions.

Neither of these known methods is applicable to clays having a silicic acid content higher than 40%,. Thus, certain clays of preferably Vmuchl'ower silica content, have been `heated `to temperatures between 400- and 700 C. and have -then been extracted vwith boiling concentrated solutions of alkali met-al hydroxides, ,which treatment results fin a preferential extraction of the silica, leaving a residue richer in alumina `and poorer in silica than the original material. i

However, -in the case of a large `groupof clay materials, such as kaolins as well as pyrophyl'lites `and certain montmorillonites, all of which materials have an SiO2 content above 40%, heatingto temperatures between 40.0 and 700 does not have any marked effect on the solubility of the silicic acid contained l,in these materials. SiO2 is present in these materials in compounds whichk are dithcultly soluble or completely insoluble in alkali solutions (sodium hydroxide or carbonate). Heat treatment below 800% C. converts only about 5 to 7.5% of this silica con.- .tent vinto a soluble form.

Furthermore, about 'i2 -to `4% of the alumina contained lin the 'heated Amaterial are made soluble, so that the ratio of soluble SiO2 4to soluble A1203 in the `material treated at temperatures below 8009 C. is about 2:1, which is 2,939,764 Patented June 7, '1960 ECC unduly high `in View of the fact that it is desired to dissolve a maximum amount o f silicic acid, but restrict vthe dissolution of alumina to a minimum or, preferably, suppress the same completely.

When speaking of silica or silicic acid or SiO2 content, this is to be understood as meaning that silicon is present bonded to oxygen in these materials in an amount which, in conventional quantitative chemical analysis, would be `calculated as percent of SiOg.

Due to rather involved chemical reactions constituting a thermic cleavage and recrystallization of the treated materials, calcination of the same Vat temperatures above 800 C. and up to 1100 C. leads to the conversion of much higher proportions of the total SiOg content into soluble form. s

When calcining `alumina containing materials of various SiO; contents at higher temperatures (800l'100 C.) the ratio of soluble silica to soluble alumina is greatly improved, i.e. ra considerably higher rate of increase in silicic acid solubility is accompanied by a considerably lower rate of increase in the solubility of alumina.

Now, in the case of clays of different SiO2 contents, experiments have been made in the past to find out, whether calcinaticn and subsequent treatment with caustic soda (NaOH) solutions of low concentrations would permit removal of the silicic acid from the clays in sufficient proportions to obtain suitable starting materials for the lime-sodaior the Bayer process.

It was found by these earlier .experiments that calcination of the clays .at `temperatures about l000 C. followed by an extraction treatment with caustic soda solutions having a concentration of about 10% NaOH permits theremoval of substantial amounts of silicic acid from the clays, but at the best it was possible thereby to obtain an alumina-enriched product in which the molar ratio of SiO2:Al2O3 was 1:1, `which corresponds to the composition of sillimanite. Such enriched materials have an alumina `contentof 45 to 52%, but also `still contain 30 to 25%, respectively, of SiO2. AWhen a more highly concentrated sodium hydroxidesolution was used, it was found that the solubility of alumina in the caustic soda increased more rapidly, while the solubility rate of increase of silica appeared to be reduced.

Thus, when using sodium hydroxide solutions containing 15% NaOH, and comparing the extraction results with those of solutions containing 5 and 7.5 and 10% `NaCl-I, it was found that the proportion of the total SiOg content that is soluble did indeed increase from about 45% to about 69%, but also that the solubility of A1203 increased trom about 1.5-2% (with solutions of 5 and 7.5% 0f NaOH) to about 7.5% of the total A1203 content of the extracted material, i.e., while the soluble content of SiO2 was not even doubled, the content of soluble alumina `increased about four to five times.

These discouraging results led to the belief 'among those skilled in the art that increases in the NaOH concentration of extraction lyes for calcined clays would lead to losses of alumina in the order of 415% and higher. Consequently the marked attack on the alumina in the product was considered so prejudicialv to the economy of the process that the above sillimanite type product was considered the optimal one that could be attained, if clays, and in particular clays having a higher SiOg content than 40%, were to be used as starting` materials.

It is, therefore, an object of our invention to provide a method for the lproduction of alumina enriched materials to be used as starting materials for decomposition processes of the above-mentioned types leading yto the production of alumina hydrates, which method permits an increase in the alumina content of clays having an SiOg, content above 40% "so as to obtain an enrichedk .This .object is attained by the method according. to the .invention which is'based on our discovery that, contrary -to .the hitherto held beliefs, the solubility rate of alumina does not further increase but remains stationary or even .slightly decreases when extraction of.` the clays having a .high silicic acid (about 40%) which have been pre- .'calcined at 900. C. to 1100 C. and preferably at 1000 C., is carried out with sodium hydroxide solutions having an NaOH content above 20%. and ata .temperature between about 170?. C., and. the boiling temperature of the NaOH solutions preferably at about .100 C., and. for a relatively short time of vto 30 minutes. We further 'ifound that theproportion of thesilicic acid content of the '.clays, that .canbe removed .withsuchvsodium hydroxide solutions of higher concentration has a maximum. range ...when thesolutionshave. a NaOH. contentbet-ween and about 35 to 40%.. .i

4The nature ofour discovery.v and the method accord- .fing to the invention shall be illustratedhereinafter with `the aid of a number of graphs andflowsheets contained in the accompanying drawings, in which .Figure 1. .is a graph illustrating the .proportions of silica and .aluminacontents of vclays having an Si02 con- -tent above..40.%, dependent upon thev calcination temper- .-ature applied..

. FigureZis a. graph showing the solubility rate of the ',SiOg and A1203 of aclay having `a .SiQapontent higher than 40% and calcined at about .1000 C. as a function of. the NaOH concentration of theextraction. agent.

.- Figure 3 illustrates in a graph theproportions of the .carrying out the method according to the invention.

. The method according to .theinvention shall now be .described in detail with the aid of the` flow-sheet o f.Figure.4.

Suitable starting materials for the pyroce'sslaccording to the inventiouare claysvhaving a Si02.content higher than .40%, comprising minerals Yof the kaolin group having .the basic formula -such as nakrite, dickite and hre-clayfand other halloysites of the general formula o v` A1203 .2Si02. 11H20 Y furthermore minerals of the pyrophyllite group having the general formula Al,o,.4sio,.H2o

and clay constituents such as montmorillonite having the formula I All these materials shall be comprised hereinafter unlder the term clays, and have usually an Si02 content between 45 and 55%. l y f One of these starting materials or a mixture of several of them is rst treated, according to Step I, as illustrated inthe flowsheet of Figure 4, by calcination in a conventionalfurnace at temperatures' between 900 and 1100" C.,'an d preferably at about 1000 C.

We have found ity to be'of advantage to restrict the cal- Vclnatlon. step to about one hour.` We have' found that calcmatlon at about 1000 C. for about one hour is sufclent to-achieve a complete thermic cleavage and recrystalllzations in the treated material so as to achieve the ymaximum attainable rate of conversion from difiicultly soluble or insoluble silicic acid contained in the 4clays. `to a type of SiOg compounds which are easily soluble in the alkaline extraction agent according to the invention.

ln this case, it is possible to convert about of the total SiOz contentt'o `the soluble form, while only about 8% of the A1203 present in the clays, becomes soluble, i.e. the ratio between the soluble shares of SOZIA1203 At temperatures below 1000 C. and in particular 900 C. the conversionv of SiOz is incomplete, as will be seen from the graph of Figure l.

It is also clear from this graph that, when calcining the clays suitable as starting materials according to the invention, the upper temperature limit is also critical, and that already at about 1100 C. and still more at higher temperatures, the conversion rate of the SiO-,- content to the soluble form decreases rapidly.

Our above-mentioned discovery. that this conversion, when taking place at 1000i C. is already complete after about one hour, is of great economical importance. Fur- `in particular that, in accordance with our' discovery, the

solubility of the alumina in the calcined material .re-

`mains surprisingly enough rather constant and lbelow about 8% and appears to b e independent of changes in the concentration of the extracting agent above an NaOH- content of 15% after an initial doubling of the alumina solubilityrate with an increase of the NaOH content 'i the extraction agent from 7.5 to 15 Figure 2 further illustrates our discovery that a maximum removal of soluble SiOZ from the calcined clay can be achieved with a sodium hydroxide solution containing about 30% of NaOH. In order to achieve these results, the sodium hydroxide solution should have a temperature between about 70 C. and its boiling temperature, preferably about C., and the timeof extraction should be limited to about 15 to 30 minutes.

It is essential in carrying out this step of the method invented by us, that the time of treatment of the clays havmg a high silicic acid content should only be very short and as a rule within the above stated time limits,

since we found that in this manner it is possible .to disried out with the sodium hydroxide solution of the above mentioned optimal concentration and within the specified exceeding 8% may become dissolved. Y l

The improved process according to the invention thus overcomes the objections raised by those versed in the yart to the use of higher concentrations of caustic soda solution for the removal of silicic acid from clay having a high silicic acid content.

As the next following step (HI) in the method according to the invention, the extracted calcinate product 1s then separated by filtration in a conventional manner from the sodium-silicate containing extraction lye.

According to a preferred mode of operation illustrated in the iiowsheet of Figure 5, the extracted and ltered calcinate residue is `subjected to a second .extractionand time limit, while only a small amount of alumina not washing step ('IV) with amore dilute aqueoussodiuni hydroxide solution having a concentration not exceeding and preferably between 5 andV 7.5 of NaOI-Lunder the same conditions of temperature and time as the first extraction step. No excess pressures are required through'- out the process. i

By the various process steps described hereinbefore, it is possible to remove 80 to 85% of the initial 4Si02 contents from clays having a high silicio acid content above 40%, while normally lese .than 8% and maximally 10 to 15% of the alumina initially contained in the clays, is dissolved.

The amounts of alkali metal hydroxide (cansic soda; or caustic potasli, although the yuse of -tl 1e latter will hardly ever be practical for economic reasons) needed for the two extraction stages are about 0.5 to 2,0 moles of Na for each mole of Si02;A a molar ratio of about 1:1 is preferred. However, ahigher rate of alkali would be excessiveand reduce the economy of the process.

A further feature of lthe invention consists" in treating the lye having a high sodiumsilicate content produced in the first, or the rst and second extraction step of the process, and then separated IbY ih? filtration step or steps, with lime and/or`I 1 1agr 1esia,` or their hydrates, thereby freeing the lye from silica and re-caustifr'hying it, whereupon the lye may be recycled to .the extraction steps.

The final product may be termed an artificial bauxite for it contains an Si02 content of 15 to 20% and even less, while its alumina content exceeds 65%.

Figure 3 shows a graph illustratingthe conversion of a clay having an initial Si02 content of about 45% and an initial A1202 content of about 3,5% to an artificial bauxite having an Si02 content of about 20% and an A1203 content of 69%.

A11 percentages in this application are `given by weight. Cerrespendnsly, the increase inthe percentages 0f beth S102 and A1203 during the calcinatio .steparedue tothe loss of water from clay, and at the higherA temperatures also due to evaporation of other impurities (C02 from carbonatos etc.).

The content of SiO2 in the artifcal bauxite can be further reduced when using as a starting material mixtures of clays and bauxites.

The invention will be further illustrated by a number of examples given below which are, however, not meant to be limitative in any way.

EXAMPLE I 19.9% of Si02 and 70.5% of A1203 EXAMPLE H A clay containing 51.2% of S102 and 43.7% of A1203 is treated in the manner described in Example I, thus producing a rst residue containing 28.8% of SiO2 and 65.1% of A1203, and after the second treatment a product containing 19.6% of Si02 and 69.6% of A1203.

EXAMPLE III With another clay, containing 51.3% of Si02 and l 43.7% of A1203, a first residue containing 20.6% of Si02 and 70.5 of A1203 was obtained, while the final product contained 17.3% of S102 and 72.2% of A1203.

The following tables will show the results obtained by a series of further examples made according to the method o f the invention, and by repeating Example with dierent materials.

A further `series `of examples were carried out with mixtures in varying ratios `of a `clay `containing 53.80 Si02 and 41:70 A1203 and a bauxite containing 21.20 S102 and 51.20 A1203.

Table II MIXTURES OF A EOVE CLAY AN'D 'B AUXITE StartingMaterial Final Product Example No.'

4Percent Percent Percent Percent ein anni@ Sigi i 4h91 It will he. understoetthatfwhile there have been given herein eertaii 1.speeiiie exemples ef the fnraetiee 0f this invention, it is not intended thereby to have this in ntien limited .te 0r eireiimseribed fby "the Sneeitie details ef materiels. yprepottiene er .conditions herein speeied in view .of thezfaet that this inventiea tney tbe modified according to individual preference or conditions without necessarily departing from the spirit of this disclosure and the scope of the appended claims.

What is claimed is:

1. A method for reducing the silica contents of alumina-containing materials and simultaneously increasing the content of alumina, comprising (a) calcining an alumina-containing starting material selected from the group consisting of kaolinites, pyrophyllites and montmorillonites, having SiO2 contents of at least about 40% by weight, at a temperature of about 900 to 1100 C. for about one hour, so as to convert a major portion of the Si02 present therein into silicate soluble in sodium hydroxide solutions containing about 20% by weight of NaOH, While less than about 10% of the alumina contained therein have become soluble even in strong alkaline solutions, thereafter (b) dissolving said soluble silicate from the calcined material with a sodium hydroxide solution containing above 20% of sodium hydroxide at a temperature of between C. and the boiling temperature of the said solutions, and for a period of about fifteen to thirty minutes, and then (c) separating the silicate-containing solution from the undissolved residue containing the aforesaid increased rate of alumina and the reduced rate of silica.

2. A method as described in claim 1, characterized in that a mixture of a clay selected from the group consisting of kaolinites, pyrophyllites and montmorillonites, having Si02 contents of at least about 40% by weight, having a high Si02 content of at least about 40% and a bauxite having an Si02 content above 10% by weight, is used as the alumina-containing starting material.

3. A method of reducing the silica contents of aluminacontaining materials and simultaneously increasing the content of alumina, comprising (a) calcining an aluminacontaining starting material selected from the group consisting of kaolinites, pyrophyllites and montmorillonites, having SiOz contents of at least about 40% by weight, at a temperature of about 900 to 1100" C. lfor about one hour, so as to convert a major portion of the Si02 present therein into silicate soluble in sodium hydroxide solutions vcontaining above 20% by weight of NaOH, while less than about of the alumina contained thereinl have become soluble even in strong alkaline solutions,vthereafterh(b) dissolving themajor portion of said soluble silicate from the calcined material with a sodium hydroxide solution containing' above 20% of sodium hydroxide at a temperature of between 70 C. and the boiling temperature of Athe said solutions, vand fora period of about fifteen to thirty minutes, then (c) separating the Asilicate-containing solution from the un; dissolved residue containing the aforesaid increased rate .of alumina and thereduced rate of silica, and (d) dissolving the portion :of soluble .silicate still retained Vin the separated residue by leaching out the latter- Witha sodium hydroxide solution containing from 2 to 10% of sodium hydroxide. v g y i 4. A method for reducing the silica contents of aluminacontaining materials and simultaneously increasing the content of alumina, comprising (I) calcining an aluminacontaining starting material selected from the group consisting of kaolinites, pyrophyllites and montmorillonites, having Si02 contents of at least about 40% by weight, at a temperature of about 900 to 1100 C. for about one hour,Y so as to convert a majorrpart, ofgrthe SiOz present therein into silicate soluble in sodium hydroxide solutions containing above vby weight of'NaOH, `while lessl than about 10% of the alumina contained therein have become soluble even in strong alkaline solutions, thereafter (II) dissolving saidsoluble silicate from the calcined material with a sodium hydroxide solution containing above 20% of sodium hydroxide'V at a temperature of about 70 C. and for a period of about fifteen to thirty minutes,'then (III) separating the silicate-v containing solution from the undissolved residue vcontaining the aforesaid increased rate of alumina and the reduced rate o f silica, and (IV) treating the separated silicate-containing solution with an alkali earth metal oxide .hydrate so as to vprecipitate ,SiOz therefrom, and recyclingthe resulting caustic solution` into stage (Il).

5. A method for reducing the silica .contents of aluminacontaining materials and simultaneously increasing the content of alumina therein, comprising (a) calcining an alumina-containing starting material lselected from the group consisting'of kaolinites, pyrophyllites, and montmorillonites, havingSiOa contents of at least about 40% by weight, at a temperature of about 1000 C., for about one hour `thereby converting about at least of theSHiO, present therein into silicate soluble in sodium hydroxide solutions containing about 30% by weight of NaOH, while Yless than about 10% of the alumina contained therein are made `soluble in sodium hydroxide solutions, lthereafter (b) dissolving said soluble silicate froml the, calcined material with a sodium hydroxide solution containing Aabout 30% of sodium hydroxide at a temperature of between 70 C. and the boiling temf perature ofthe said solutions, and for a period of about fteen'to thirty` minutes, and then (c) separating the silicate-containing solution from the undissolved residue containing an increased rate of about at least of the alumina and a reduced rate of less than 30% of the silica present in the aforesaid starting material.

References Cited in the file of this patent kUNITED STATES PATENTS OTHER REFERENCES U.S.` Bureau of Mines Publication, Rl. 4132, November 1947,'Recovery Process, by Brown, R. A. et al., pages 6,712', 13, 20, 6l. 

1. A METHOD FOR REDUCING THE SILICA CONTENTS OF ALUMINA-CONTAINING MATERIALS AND SIMULTANEOUSLY INCREASING THE CONTENT OF ALUMINA, COMPRISING (A) CALCINING AN ALUMINA-CONTAINING STARTING MATERIAL SELECTED FROM THE GROUP CONSISTING OF KAOLINITES, PYROPHYLLITES AND MONTMORILLONITES, HAVING SIO2 CONTENTS OF AT LEAST ABOUT 40% BY WEIGHT AT A TEMPERATURE OF ABOUT 900 TO 1100*C. FOR ABOUT ONE HOUR, SO AS TO CONVERT A MAJOR PORTION OF THE SIO2 PRESENT THEREIN INTO SILICATE SOLUBLE IN SODIUM HYDROXIDE SOLUTIONS CONTAINING ABOUT 20% BY WEIGHT OF NAOH, WHILE LESS THAN ABOUT 10% OF THE ALUMINA CONTAINED THEREIN HAVE BECOME SOLUBLE EVEN IN SOLUBLE SILICATE SOLUTIONS, THEREAFTER (B) DISSOLVING SAID SOLUBLE SILICATE FROM THE CALCINED MATERIAL WITH A SODIUM HYDROXIDE SOLUTION CONTAINING ABOVE 20% OF SODIUM HYDROXIDE AT A TEMPERATURE OF BETWEEN 70*C. AND THE BOILING TEMPERATURE OF THE SAID SOLUTIONS, AND FOR A PERIOD OF ABOUT FIFTEEN TO THIRTY MINUTES, AND THEN (C) SEPARATING THE SILICATE-CONTAINING SOLUTION FROM THE UNDISSOLVED RESIDUE CONTAINING THE AFORESAID INCREASED RATE OF ALUMINA AND THE REDUCED RATE OF SILICA. 